1. Field of the Invention
This invention relates to articulated fluid transferring apparatus, and more particularly to apparatus for determining the spatial position of the outer end of marine loading arms and for disconnecting such arms from a floating vessel when a computer predicts that the outer ends of such arms may move into a danger area.
2. Description of the Prior Art
Fluid loading arms constructed of articulated pipe are extensively used in the petroleum industry for transferring oil or other fluids between a buoy or other loading terminal and a marine tanker. Such arms generally comprise an inboard limb boom supported on the buoy or loading terminal by a pipe swivel joint assembly to facilitate pivotal movement about horizontal and vertical axes, and an outboard limb pivotally connected by a pipe swivel joint to the inboard limb or boom for movement relative thereto about a horizontal axis. The outer end of the outboard limb is adapted to be connected to a pipe manifold on a tanker located within reach of the arm, such as by a remotely-controllable coupler device.
When an installation of this type is being designed, minimum requirements are set for the reach of the arm. These requirements are expressed in terms of the maximum horizontal displacement of the tanker parallel to and away from the buoy relative to a datum position, the maximum displacement away from the buoy due to variations in distance between the tanker manifold and the tanker rail, and the maximum vertical displacement due to variations in the water level and the height of the tanker manifold relative to the water level.
These displacements define a three-dimensional space that is rectangular in section when viewed in plan or in elevation, either parallel to or perpendicular to the jetty, and the space is known as the arm's "operating envelope". The arm must be able to accommodate all of these displacements so that a safe and secure connection to the tanker's manifold can be established and maintained within the limits of this envelope.
Most articulated arms are counterbalanced so that when empty they are substantially self-supporting. However, the weight of the oil or other fluid in the arm during use is not counterbalanced and thus must be supported in part by the tanker manifold to which the arm is connected. Clearly, the stress on the manifold increases with the extension of the arm. In addition, the manifold always faces toward the tanker rail, and the stress to which the manifold can be subjected in a direction perpendicular to the rail, and hence to the jetty, is greater than the stress to which it can be subjected parallel to the rail. The stress parallel to the rail increases with an increase in the slew angle, that is the angle between the vertical plane in which the arm resides and the vertical plane through the riser and normal to the edge of the jetty. Thus, to prevent the stress on the manifold from exceeding safe limits, the extension of the arm and the slew angle must be limited.
To achieve this limitation, alarm systems have been provided for actuation in the event of the angle between the inboard and outboard limbs exceeding a predetermined limit, or in the event the slew angle exceeds a predetermined limit. These prior art systems are not entirely satisfactory as the outboard end of the arm may continue to move beyond the safe limit and the manifold may be damaged before fluid flow through the arm can be stopped and the arm disconnected from the tanker manifold. A more satisfactory system would be to use the movement of the outer end of the loading arm to predict when the arm may move outside the safe area, and to start a shutdown procedure before the arm reaches the danger area.